Walton, "The Ultrasonic Gas Analyser" (1952) discloses a gas analyzer in which a pair of ultrasonic transducers are placed at opposed ends of an elongated sample tube which is "many wavelengths" long. The transmitting transducer is coupled to an oscillator, and the receiving transducer is coupled to a phasemeter which receives a reference input from the oscillator. The phasemeter provides a continuous indication of velocity of radiation in the tube, which in turn varies with impurity concentration. However, the apparatus is subject to inaccuracy due to variations in gas temperature, which also affects radiation velocity. In order to overcome this problem, apparatus is proposed in which a second tube containing a "standard gas" is connected between the oscillator and phasemeter to provide a reference which compensates for temperature variations in both tubes. Hasewell, et al. "Development of the Sonic Gas Analyzer" describes a two-tube instrument of this character marketed by Grubb Parsens in about 1961.
Although instruments of the above-described character can be employed to advantage in a laboratory, the size and complexity of such instruments are not well suited to field use. In applications for detection of refrigerant leaks as in a building air-conditioning system, for example, it would be difficult to manipulate the box containing the two tubes into relatively inaccessible areas in which the refrigerant lines are normally run. Furthermore, the volumes of gas in the sample and reference tubes greatly reduces response time. High voltage corona discharge detectors of the type disclosed is U.S. Pat. Nos. 3,742,475 and 4,488,118 can be more readily manipulated into relatively inaccessible areas, but suffer from other drawbacks.
A general object of the present invention is to provide a device of the described character which is inexpensive to manufacture and reliable in long-term operation, which is contoured and constructed to be held in the hand of an operator and readily manipulated by the operator for locating refrigerant leaks in confined areas, which exhibits reduced battery power consumption, which provides both audible and visible indication of detected contaminant conditions, which automatically accommodates gradual variations in background or ambient contaminant level, and which eliminates any requirement for high voltage at the probe as in corona discharge apparatus of the prior art.